1. Field of the Invention
The present invention relates to a surface light source device of side light type and more particularly relates to a surface light source device of side light type in which uniformity of luminance is improved. The present invention is applied, for example, to backlighting for a liquid crystal display.
2. Related Art
A surface light source device of side light type is conventionally applied to a liquid crystal display to illuminate a liquid crystal panel from its back surface. This arrangement is suitable for making an entire shape of the surface light source device thin.
A bar-shaped light source such as a cold cathode tube, etc. is generally adopted as a primary light source in the surface light source device of side light type and is arranged on the side of a guide plate (a guide material formed in a plate shape). Illuminating light emitted from the primary light source is introduced into the guide plate through a side end surface (incidence surface) of the guide plate. The introduced illuminating light is propagated within the guide plate and light is emitted from a major surface of the guide plate toward the liquid crystal panel during this propagation.
It is known that the guide plate adopted in the surface light source device of side light type is of a type having a substantially uniform plate thickness and a type having a tendency to reduce in thickness corresponding to the distance from the primary light source. In general, the latter efficiently emits illuminating light in comparison with the former.
FIG. 12 is an exploded perspective view showing a conventional surface light source device of side light type using the guide plate of the latter type. FIG. 13 shows a cross section taken along line A--A of FIG. 12. With reference to FIGS. 12 and 13, the surface light source device 1 of side light type has a guide plate 2, a primary light source 3, a reflection sheet 4 and a prism sheet 5 serving as a light control member. The reflection sheet 4, the guide plate 2 and the prism sheet 5 are laminatedly arranged. The primary light source 3 is arranged on a side of the guide plate 2.
The guide plate 2 is made of a scattering guide material having a wedge-shaped cross section and is called a scattering guide plate. For example, the scattering guide material is made of a matrix formed of PMMA (polymethyl methacrylate) and many transmissive fine particles uniformly dispersed into the matrix. Refractive index of these fine particles is different from that of the matrix.
The primary light source 3 has a cold cathode tube (fluorescent lamp) 7 and a reflector 8 having a nearly semicircular shape in cross section and backing the cold cathode tube 7. Illuminating light is supplied to an incidence surface 2A as a side end surface of the scattering guide plate 2 through an opening of the reflector 8. A sheet-like regular reflection member made of a metallic foil, etc., or a sheet-like irregular reflection member made of a white PET film, etc. is adopted for the reflection sheet 4.
Illuminating light is introduced into the guide plate 2 through the incidence surface 2A and is propagated toward a distal end of the guide plate 2 while this illuminating light is repeatedly reflected between two major surfaces (a back surface 2B and an emission surface 2C). In the meantime, illuminating light is subjected to scattering action by the fine particles within the guide plate 2. Irregular reflecting action of illuminating light occurs when the reflection sheet 4 made of an irregular reflection member is adopted.
Incident angle of illuminating light to the emission surface 2C is reduced stepwise after every reflection on the inclined back surface 2B. The reduction in the incident angle causes an increase in light components having an angle equal to or smaller than critical angle with respect to the emission surface, and promotes light emission from the emission surface. Thus, deficiency of the emitted light in an area far from the primary light source 3 is prevented.
Illuminating light emitted from the emission surface 2C has a property of scattering light since this illuminating light is scattered by fine particles within the light scattering guide plate 2 or is further irregularly reflected on the reflection sheet 4. However, a main propagating direction of illuminating light from the emission surface 2C is inclined in a distal direction (a direction in which illuminating light goes away from the incidence surface 2A) with respect to a frontal direction on a surface parallel to the incidence surface 2A. Namely, the emitted light of the scattering guide plate 2 has directivity. Such property is called directivity emitting property.
The propagating direction of illuminating light from the emission surface 2C is symmetrically spread on both sides with respect to the frontal direction on a plane perpendicular to the incidence surface 2A. Namely, light components emitted in left-hand and right-hand slanting directions are included in illuminating light as seen from the incidence surface 2A.
The prism sheet 5 arranged along the emission surface 2C is made of a transmissive sheet member such as polycarbonate, etc. The prism sheet 5 has a prism surface on which many parallel prism rows are formed. In this example, the prism sheet 5 is oriented so that a prism surface is directed to the guide plate 2 and the prism rows approximately extend in parallel with the incidence surface 2A.
For example, each of the prism rows is made of fine projection rows having a triangular shape in cross section. Slopes of these projections correct the obliquely emitted illuminating light in a frontal direction on a surface perpendicular to the incidence surface 2A. A so-called double-sided prism sheet having prism surfaces on both sides thereof may be used. In this case, the prism rows on an outside prism surface run in a direction approximately perpendicular to the prism rows on an inside prism surface. The outside prism surface corrects illuminating light in the frontal direction on a surface parallel to the incidence surface 2A.
In the above surface light source device 1 of side light type, it is supposed that utilization efficiency of illuminating light can be improved by integrating the prism sheet 5 and the scattering guide plate 2.
However, when the prism sheet 5 and the light scattering guide plate 2 are practically integrated, a reduction in luminance level is observed along both sides from a portion near the incidence surface as shown by an arrow B in FIG. 11. This reduction is not desirable since it reduces quality of illuminating light.